What if Julian the apostate had an heir?
A sort of universal secreisity. Christ came earlier as Heracles, balder. The avatar concept.
Really think that was far too late to hold back the Christian flood tide.
As noted, too many elite functionaries of the Roman system were Christian converts; at this point what they wanted was for the state to adopt it and settle its social message ambiguities in favor of the powers that be, and too many Christian apologists had outlined how such a cult could be useful to a Roman state in dire need of stabilizing forces. At this point I suppose a real irritant remained the country/"cosmopolitan" (as I characterize both the city centered local elites and the wide ranging traders) divide, but no cult the anti-Christian minority of elites liked would have particular traction on the country side of this polarization either.
What is needed is something much earlier that can soak up the "seekers" of various classes and unify them, then even if whatever events in OTL actually happened in Judea to underly OTL Christian canon had happened just the same, as they might well not of course, the Christians are just one more quaint and splintered bunch of cultists displaced by the ATL thing.
"Balder" I suppose is a reference to the Nordics, who OTL are pretty much unknown to Mediterranean classical scholarship at this early date. It may be that the magnitude of transmission of Mediterranean high culture to the Baltic was a lot more than people guess, and so if some powerful ATL thing were happening in the south, then it might have an effective echo in the North too. But again some small faction of anti-Christian dissidents in Julian's day will not cut it, not likely even if we imagine a movement for them to flee the Empire and run north to establish themselves in the far north. Not likely the Northern peoples around the Baltic will accept them on anything more than a personal basis as exiles for one thing! If they can manage even that!
"The avatar concept..." maybe indeed some ATL extra push out of India can soup up some known OTL cult or synergize a new one that can interpose and as noted, soak up the "seekers" before the Christians do that, and then who knows, it depends on details.
Already put out my favored Zoroastrian notion, which I suppose might happen even without a Pontic patron. Say some Roman dynasty of Emperors are especially taken with some Persian derived cult. Mithraism is often offered as an alternative, but I think as offered, it is too limited, being a cult exclusively for men and particularly for soldiers. That's well and good for capturing the militarized elite men, but rotten at the sort of grassroots penetration and permeation Christians accomplished by converting women and having mothers raise their kids in their own faith. So there is still room I suppose for a major mutation of Mithraism or a rival cult...as long as it has that ability to permeate the women and the slaves and other poor folk. And at the same time attract a bunch of Hellenistic philosopher types to harmonize it with Classical high culture.
If you don't like the Persian lineage, and I can respect that since I was largely shooting from the hip when I first elaborated this "Hellenistically generalized Ahurism" concept, and some detailed reading on classic forms of Zoroastrianism suggest some major pitfalls a mutant version would have to leapfrog, try your own hand at some Buddhist or Vedic derived thing if you like. Or even take a leap with quite other roots such as elaborating Isis or something like that, or a Romano-Celtic synergy.
Again, what is needed is 1) deep appeal, if not to traditional peasantries that remain rooted to their traditional grassroots paganism, instead to the "lazarus stratum" of the cosmopolitan empire or ATL empires, urban poor and women especially I think, for depth and deep grassroots, albeit maybe not in the countryside; 2) appeal to upper class Seekers who find the traditional paganism too limited and rustic and want to square their mythic energies with respectable Hellenistic rationalism and their cosmopolitan experiences.
Note that we can trade off some of 1) for broader "pagan" appeal since the name of the game is precisely to reduce that conflict and foster lots of culturally rooted extensions of old pantheons. But not too much; it has to say something to at least some of those low class people or the cities will be too hard to hold.
We might wind up with an ongoing polarization in which the urban power elites consciously value their countryside hold and are prepared to repress the urban masses. Demographically, urban areas tended to be power centers but also demographic sinks; country life tended to enable those who matured to adulthood to be healthier and stronger, and death rates tended to outrun birth rates in cities so that urban populations were always being renewed by migrants from the countryside.
A regime conscious of that might develop political strategies to keep the urban masses under their thumbs and count on recruiting soldiers from the countryside to do it, but I think that was kind of tricky to manage, as urban elites would have interests that tended to work against countryside prosperity. It was kind of a push between "maintaining order" hence countryside peace and security for traders, versus the one-way tendency for exploitation to sink its teeth into these rural stationary targets--any countervailing mobility of the country folk would work against imposed order. I suppose this is one of the factors behind evolving from Classical era centralized empires and kingdoms to medieval feudalism and manorialism, shifting the effective power base from city to country, and Classical culture showed little ability to stay ahead of that curve.
So maybe an ATL country rooted paganism might catalyze the transition from urban centered to rural centered power systems?